With semiconductor devices, for example, large-scale integrated circuit devices typified by DRAM, etc., there is an eager demand for even greater density, more integration or higher speed. In accordance with these trends, demands for the establishment of half-micron microfabrication technologies, for example, the development of photolithography technologies for the formation of fine patterns, have become increasingly intense in the field of production of various electronic devices. To form fine patterns by the photolithography technologies, it is necessary to increase resolution. The resolution (R) of a reduced projection exposure system is represented by Rayleigh's equation R=k·λ/NA (where λ represents the wavelength of exposure light, NA represents the numerical aperture of a lens, and k represents the process factor. By shortening the wavelength λ of active radiation (exposure light), which is used in forming a resist pattern, therefore, the resolution can be increased.
As photoresists suitable for short wavelengths, those of the chemical amplification type are proposed. The chemically amplified photoresists are characterized in that a proton acid is generated from a photoacid generator, which is a component contained therein, by irradiation with exposure light, and this proton acid causes an acid catalyst reaction with a resist resin or the like upon heating after exposure. Most of the photoresists which have been developed currently are of the chemical amplification type.
As acids generated upon exposure from photoacid generators, alkanesulfonic acids or partially or completely fluorinated alkanesulfonic acids are used.
The alkanesulfonic acid-generating photoacid generator generates an acid whose strength is weak. Thus, a protective group which facilitates deprotection is introduced into the resin for use in the chemically amplified resist, and scaling-down of designs is considered. To use the protective group whose deprotection is easy, a sulfonic acid having a large molecular size and low diffusibility, such as camphorsulfonic acid, has been used as an effective sulfonic acid. However, the use of the sulfonic acid with low diffusibility requires a large amount of the acid generated and, as a result, poses the problems that the amount of exposure is large and the productivity decreases.
Acid generators which generate completely fluorinated alkanesulfonic acids have sufficient acid strength against a deprotection reaction for a protective group whose deprotection is difficult, and most of these acid generators are in practical use. However, too high an acid strength thereof causes an unexpected reaction in the elimination reaction of the protective group for reversing the dissolution contrast of the resin, thereby arousing the problem that foreign matter occurs after alkali development or during resist stripping. Patent Document 1 reports that the problem of presenting foreign matter has been solved by using a publicly known sulfonic acid having moderate acid strength in which the alkyl group of the alkanesulfonic acid has been partially replaced by a fluorine atom, a nitro group or the like being an electron attractive group. With a compound which generates a sulfonic acid having 3 or more fluorine atoms, however, foreign matter occurs after alkali development or during resist stripping, leading to unsatisfactory results. Patent Document 2 reports that moderate acid strength is imparted, without occurrence of foreign matter, by using a compound generating a sulfonic acid in which an alkyl group and a perfluoroalkyl group have been introduced into the alpha-carbon atom of methanesulfonic acid. However, sufficient acid strength has not been obtained. Patent Document 3 discloses a sulfonic acid having high acid strength, but sufficient properties have not been obtained in connection with the occurrence of foreign matter.